Although improvements in aircraft and engine technology and in the efficiency
of the air traffic system will bring environmental benefits, these will not
fully offset the effects of the increased emissions resulting from the projected
growth in aviation. Policy options to reduce emissions further include more
stringent aircraft engine emissions regulations, removal of subsidies and incentives
that have negative environmental consequences, market-based options such as
environmental levies (charges and taxes) and emissions trading, voluntary agreements,
research programs, and substitution of aviation by rail and coach. Most of these
options would lead to increased airline costs and fares. Some of these approaches
have not been fully investigated or tested in aviation and their outcomes are
uncertain.

Engine emissions certification is a means for reducing specific emissions.
The aviation authorities currently use this approach to regulate emissions for
carbon monoxide, hydrocarbons, NOx, and smoke. The International Civil Aviation
Organization has begun work to assess the need for standards for aircraft emissions
at cruise altitude to complement existing LTO standards for NOx and other emissions.

Market-based options, such as environmental levies (charges and taxes) and
emissions trading, have the potential to encourage technological innovation
and to improve efficiency, and may reduce demand for air travel. Many of these
approaches have not been fully investigated or tested in aviation and their
outcomes are uncertain.

Environmental levies (charges and taxes) could be a means for reducing growth
of aircraft emissions by further stimulating the development and use of more
efficient aircraft and by reducing growth in demand for aviation transportation.
Studies show that to be environmentally effective, levies would need to be addressed
in an international framework.

Another approach that could be considered for mitigating aviation emissions
is emissions trading, a market-based approach which enables participants to
cooperatively minimize the costs of reducing emissions. Emissions trading has
not been tested in aviation though it has been used for sulfur dioxide (SO2)
in the United States of America and is possible for ozone-depleting substances
in the Montreal Protocol. This approach is one of the provisions of the Kyoto
Protocol where it applies to Annex B Parties.

Voluntary agreements are also currently being explored as a means of achieving
reductions in emissions from the aviation sector. Such agreements have been
used in other sectors to reduce greenhouse gas emissions or to enhance sinks.

Measures that can also be considered are removal of subsidies or incentives
which would have negative environmental consequences, and research programs.

Substitution by rail and coach could result in the reduction of carbon dioxide
emissions per passenger-km. The scope for this reduction is limited to high
density, short-haul routes, which could have coach or rail links. Estimates
show that up to 10% of the travelers in Europe could be transferred from aircraft
to high-speed trains. Further analysis, including trade-offs between a wide
range of environmental effects (e.g., noise exposure, local air quality, and
global atmospheric effects) is needed to explore the potential of substitution.